A coated cutting tool, which comprises a base material comprised of a hard material having a surface coated with a TiCN columnar crystal film deposited by a chemical vapor deposition (CVD) method at a medium-temperature such as a temperature in the range of from 700 to 900° C. using raw material gas comprising CH3CN wherein the TiCN columnar crystal film is coated with an aluminum oxide film, is widely used in cutting.
As a prior art coated cutting tool, there is a surface-coated cutting tool made of tungsten carbide-based cemented carbide, which is coated with a first layer comprised of titanium nitride, a second layer comprised of titanium carbo-nitride, a third layer comprised of titanium carbo-oxide, and a fourth layer comprised of aluminum oxide (see, for example, patent document 1). However, since the surface-coated cutting tool made of tungsten carbide-based cemented carbide is coated with a titanium carbo-nitride film deposited by a medium-temperature CVD method using raw material gas comprising CH3CN, the atomic ratio of carbon to the sum of carbon and nitrogen contained in the titanium carbo-nitride film {C/(C+N)} is as small as 0.5 to 0.6. Therefore, there is a problem in that the titanium carbo-nitride coating film has a low hardness and hence does not exhibit a satisfactory wear resistance.
As another prior art coated cutting tool, there is a coated cutting tool coated with a TiCN film using raw material gas comprising CH3CN and CH4 (see, for example, patent document 2). However, when the reaction for deposition is conducted at a temperature of 900° C. or lower, only CH3CN is reacted and almost no CH4 undergoes the chemical reaction. Therefore, the resultant TiCN film does not have a C/(C+N) ratio of more than 0.6, and thus the TiCN film has a low hardness and does not exhibit a satisfactory wear resistance. On the other hand, when the reaction is conducted at a temperature of higher than 900° C., the resultant TiCN coating film is comprised of coarse crystal grains, and hence is lowered in toughness, causing a problem in that the fracture resistance is poor.
[Patent document 1] Japanese Unexamined Patent Publication No. Hei 07-328808
[Patent document 2] Japanese Unexamined Patent Publication No. Hei 06-158324